The
earliest clocks relied on shadows cast by the sun, so they were
not useful in cloudy weather or at night. They also required
recalibration as the seasons changed if the gnomon wasn’t
aligned with the Earth's axis. The first clock with an escapement
mechanism dates back to ancient Greece in the 3rd century B.C.E..
Later in the 11th century, Arabic engineers invented
water clocks driven by gears and weights.

Mechanical clocks employing a verge
escapement mechanism appeared in Europe at the turn of the 14th
century, and soon became the standard timekeeping device until the
spring-powered clock and pocket watch came on the scene in the
16th century. By the 18th century, the pendulum clock
had been invented. The 20th century brought quartz crystal
oscillators. Walter G. Cady built the first one in 1921. In 1969,
Seiko produced the world's first quartz wristwatch. Because of
their low production cost and accuracy, they have become common
possessions.

The National Bureau of Standards based
the time standard of the United States on quartz clocks from late
1929 until the 1960s, when it changed to atomic
clocks. Atomic
clocks are more accurate than any other timekeeping device, and
today clockmakers use them to calibrate other clocks.